Note: As defined by the US Census Bureau, a city's official boundaries often extend miles beyond the city itself and may even cross county lines. As such, several thousand US cities officially exist in two or more counties (and therefore appear on multiple counties' lists of cities.)
City | Rank | 2024 Pop. | 2020 Pop. | Change | Density (/mi²) | Area (mi²) | Type |
|---|---|---|---|---|---|---|---|
| Boulder | 1 | 105,156 | 108,123 | -0.7% | 3,910 | 26.9 | City |
| Longmont | 2 | 98,444 | 99,188 | -0.19% | 3,390 | 29.0 | City |
| Erie | 3 | 36,886 | 30,417 | 4.58% | 1,779 | 20.7 | Town |
| Lafayette | 4 | 30,441 | 30,433 | 0.01% | 3,298 | 9.2 | City |
| Louisville | 5 | 20,128 | 21,174 | -1.29% | 2,525 | 8.0 | City |
| Gunbarrel | 6 | 10,249 | 10,494 | 0.7% | 1,641 | 6.2 | CDP |
| Niwot | 7 | 4,105 | 4,085 | 0.17% | 1,027 | 4.0 | CDP |
| Lyons | 8 | 2,132 | 2,207 | -0.88% | 1,579 | 1.4 | Town |
| Nederland | 9 | 1,495 | 1,467 | 0.47% | 1,010 | 1.5 | Town |
| Lazy Acres | 10 | 1,030 | 1,071 | 0.78% | 195 | 5.3 | CDP |
| Paragon Estates | 11 | 845 | 785 | -0.7% | 504 | 1.7 | CDP |
| Allenspark | 12 | 638 | 464 | 1.27% | 53 | 11.9 | CDP |
| Sugarloaf | 13 | 541 | 262 | 3.84% | 248 | 2.2 | CDP |
| Altona | 14 | 536 | 479 | 0.38% | 318 | 1.7 | CDP |
| St. Ann Highlands | 15 | 479 | 486 | 2.79% | 344 | 1.4 | CDP |
| Pine Brook Hill | 16 | 444 | 776 | -7.88% | 147 | 3.0 | CDP |
| Tall Timber | 17 | 444 | 295 | 3.98% | 774 | 0.6 | CDP |
| Eldora | 18 | 421 | 352 | 4.99% | 101 | 4.2 | CDP |
| Crisman | 19 | 277 | 51 | 2.59% | 190 | 1.5 | CDP |
| Gold Hill | 20 | 252 | 195 | 0.4% | 121 | 2.1 | CDP |
| Jamestown | 21 | 241 | 255 | -1.63% | 420 | 0.6 | Town |
| Eldorado Springs | 22 | 234 | 325 | -9.65% | 91 | 2.6 | CDP |
| Sunshine | 23 | 209 | 145 | -0.48% | 121 | 1.7 | CDP |
| Mountain Meadows | 24 | 163 | 174 | -4.68% | 95 | 1.7 | CDP |
| Seven Hills | 25 | 135 | 127 | 0.75% | 276 | 0.5 | CDP |
| Bonanza Mountain Estates | 26 | 134 | 174 | 0.75% | 798 | 0.2 | CDP |
| Ward | 27 | 124 | 130 | -1.59% | 231 | 0.5 | Town |
| Bark Ranch | 28 | 115 | 107 | -5.74% | 133 | 0.9 | CDP |
| Valmont | 29 | 94 | 98 | 6.82% | 364 | 0.3 | CDP |
| Glendale | 30 | 79 | 109 | 6.76% | 63 | 1.3 | CDP |
| Hidden Lake | 31 | 34 | 30 | 9.68% | 56 | 0.6 | CDP |
Note: City markers shown outside the county boundary mean that a part of the city extends into this county, but is primarily located in an adjacent county.
Boulder is the largest city in Boulder County.
Hidden Lake is the smallest city in Boulder County.
There are 31 cities in Boulder County, Colorado.